Generally, a light-emitting device may be manufactured by using compound semiconductors of groups III and V in the periodic table as a compound semiconductor which converts electrical energy into light energy, and may implement various colors by adjusting a composition ratio of the compound semiconductors.
The light-emitting device emits energy corresponding to a band-gap energy between a conduction band and a valance band when electrons of an n-type layer and holes of a p-type layer recombine upon application of a forward voltage. This energy is usually emitted in the form of heat or light. The light-emitting device emits the energy in the form of light. For example, a nitride semiconductor has received much attention in development fields of optical devices and high-power electronic devices due to its high thermal stability and wide band-gap energy. In particular, a blue light-emitting device, a green light-emitting device, an ultraviolet (UV) light-emitting device, and the like using the nitride semiconductor have become commercialized and are widely used.
The nitride semiconductor according to the related art is formed by sequentially stacking a first conductive type semiconductor layer made of a GaN material and an active layer on a silicon substrate, and a second conductive type semiconductor layer. In order to prevent a strain from being generated due to a lattice mismatch and a thermal expansion coefficient difference between the silicon substrate and a GaN layer, a buffer layer is disposed between the silicon substrate and the first conductive type semiconductor layer.
Such a buffer layer has been mostly formed by growing AlGaN and has been formed by a method for increasing or decreasing an Al composition ratio from a surface of a substrate.
Strain control in a case where the Al composition ratio decreases from the surface of the substrate is more efficient than that in a case where the Al composition ratio increases from the surface of the substrate. However, in a case where the Al composition ratio decreases from the surface of the substrate, there is a limitation in that plastic deformation is caused by a strong compressive strain applied during the GaN growth.